Graphene has attracted great interest due to its unique electronic, thermal and mechanical properties, resulting from its two-dimensional (2D) structure, and to its potential applications like microchips, chemical sensing instruments, biosensors, energy storage devices and other technologies. With appropriate surface treatments, single graphene sheets can be separated from the graphite material and the layer-to-layer distance can be extended (DOI: 10.1021/cm0630800, DOI: 10.1016/j.fuel.2010.03.036). Graphene nanosheets can act as effective conductive fillers in polymer matrices due to the high aspect ratio, excellent electrical conductivity and cost efficiency.
One of the applicable methods is graphite oxidation in order to reduce the strong bonding between sheets in graphite and receive monolayer graphene sheet. The structure of graphite oxide (GO) resembles graphite but only difference is that the sp3 hybridization in carbon atoms and thus the individual layers are considerably bent (DOI: 10.1007/BF00943889). A very important property of graphene is its stability at ambient conditions: it can exist by being rippled rather than completely flat in a free-standing state (DOI: 10.1016/S1369-7021(06)71788-6). Therefore, electrical conductivity and surface area of graphene sheets tend to decrease due to said dimensional change of the structure.
2D graphene nanosheets have several drawbacks, especially when they are employed in bulk systems: the graphene sheets agglomerate in several polymeric matrices and solutions; the surface area and electrical conductivity of the graphene sheets decrease; graphene layers restack after reduction process; and it is difficult to process multi-layer graphene.
Available techniques in the art for synthesis of solid, core-shell type or hollow closed 3D spherical carbon bodies are as follows: arc discharge, laser ablasion and plasma techniques, shock compression techniques, chemical vapor deposition, autoclave process, microemulsion polymerization, and catalytic carbonization. In a study, hollow graphene oxide spheres were fabricated from graphene oxide nanosheets utilizing a water-in-oil emulsion technique without surfactant (DOI: 10.1039/B927302F). Cao et al. (DOI: 10.1016/j.carbon.2012.12.075) synthesized hollow graphene spheres at 160° C. for 10 h in an autoclave by hydrothermal process. In addition, template assisted chemical vapor deposition technique is used for the production of 3-dimensional graphene networks by changing the gas flow ratio and growth time. In this technique, nickel foam is used as a template and carbon source is provided at about 950° C. under Argon and H2 atmosphere and then the interconnected 3-D graphene sheets are obtained (U.S. 2012/0128573 A1, DOI: 10.1039/C1JM13418C, DOI: 10.1016/j.apsusc.2014.05.171, DOI: 10.1016/j.matlet.2014.02.077). In another study, mesoporous graphene nanoballs are synthesized by using functionalized polystyrene balls deposited catalyst under high temperature and a hydrogen gas environment during chemical vapor deposition process (DOI: 10.1021/nn401850z). Moreover, graphene hollow balls are prepared by covering polystyrene balls with graphene oxide sheets and then calcination at 420° C. for 2 h (DOI: 10.1039/C3TA12789C).
Instead of polymeric spheres, SiO2, organic oxide, is used as a template for the production of graphene hollow spheres and oxides are removed by hydrofluoric acid treatment (DOI: 10.1039/C3NR03794K). In the literature, there are also some techniques such as soft templating (colloidal template) (PMID: 10747405), hard templating (silicon template) (DOI: 10.1021/cm052219o), solvothermal technique (DOI: 10.1016/j.ssc.2004.07.004), sol pyrolysis (DOI: 10.1002/adma.200305697) and microemulsion polymerization (DOI: 10.1039/B316881F) for the production of hollow carbon spheres.
However, the main problem of graphene sheets in various shapes is the agglomeration of the sheets in polymeric matrices and solutions, which leads to a decrease in electrical conductivity and surface area of graphene sheets and affects negatively the utilization of graphene in several technological fields such as electrodes for fuel cells, supercapacitors and Li-ion batteries, catalyst supports and nanocomposite production. There are some drawbacks of available techniques for the production of 3-dimensional graphene and graphene based three dimensional bodies (e.g. in form of a sphere) which are high cost and uncontrolled size and fails in shape. A further problem is; that it is not possible to produce orderly aligned and repeating graphene spheres by applying available techniques and the production capacity is also limited.
In conventional processes, size of spheres directly depends on the template and deformation of spheres and hollowness is observed because of in-situ process.
Another problem is the stacking of graphene layers after a reduction process and an electrospinning process increases the distance between graphene layers and prevents restacking of graphene layers under electrical field. Upon an oxidation process, the change of stacking order between graphene layers and the random destruction of graphitic structure causes disorder in percent crystallinity; and it further causes decrease in surface area, electrical conductivity and mechanical properties.
Because of the characteristics of the above mentioned previous techniques, the graphene spheres obtained therewith are produced only in low amounts with high costs, since said techniques employ template-based multi-step procedures.